So 30k is picking up steam in my neck of the woods and people keep asking me to help them build a legion. So here is a quick article to help you decide. It's basically and email I shot to a friend of mine.
In 2 calth boxes gets you:
2 Termi characters
First off, any legion can really build something viable from 2 box sets. Although, legions based on infantry and termies are going to give you more a bang for your buck.
More than anything, pick a legion that you like their fluff and paint scheme and play style. Pretty much any codex can do any list, just some do certain things better than others. All the legions have a lot of different choices though, and even the legion without rules or blah rules can totally be 100% competitive.
From a cost perspective:
In general, I would decide if want to play a stay at home shooty legion or a go get someone legion. If you want to go get someone, you need transports unless you pick a legions with teleporting termies or take a ROW that gives deepstrike. As for infantry heavy legions, all the legions give you some bonus rules, but only a few increase the damage output or reduce the damage intake on infantry. For legions that don't have either things like tacs become objective sitters and you focus on other types units, which gets you back into buying stuff.
the last point, is that you will probably need to go gets some weapons or after market bits to shoehorn the calth box into some armies, but I can point you in the right direction for that. Bascially the infantry models can be turned into anything from seekers to breachers. So consider building your list before you glue the arms on your minis :)
1. Emperors Children:
Infantry: I love the purple look, but in general I don't love the EC rules.
2. Iron Warriors:
Infantry get A LOT. Shrapnel shells give all vhy bolter pinning, which is big in 30k. You are also stubborn in your deployment zone and you can rapid fire and charge as a disordered charge. Which is slightly better, but better in a bunch of situations. So IWs tend to take a lot of rapid fire weapons.
Walkers get extra armor for free. Minor, but points saved.
Tyrant termies are a legion specific coice that is very solid. They basically can all take cyclone missile launchers.
4 heavies. This is really the reason to take them. Hvy's are really good in 30k, but this will probably require you buying some extra stuff. That said, deredeo pattern contemptors are hvy and awesome. So you could run this as hvy. They also have havoc squads with are really solid tank hunters. You often see one of these squads as one of the 4 heavy options. I also think missile luanchers are your best choice with them (or autocannons or las), but missile launcher come in calth.
A standard build would be like 2 big tac squads with apothecaries and a AA sgt, a havoc squad, a scarian, a scorpius, 2 havoc squads, maybe with a HQ to give extra rules, a contemptor and maybe a defense line for cover. Or you go heavy with some machanicum castellax robots.
3. Imperial Fists:
Infantry: Are BS5 with bolter weapons, making the standard tac squad pretty scary to deal with. They also make seekers frightening, since they are bolters with special ammo. another good units is veterans, which you can unlock as troops. they can take the sniper rule on top of the BS5 making them pretty scary. Finally, hvy support squads get tank hunter, so las cannon dev squads are VERY good, and all infantry is stubborn in cover or fortifications, which is really solid in HTH.
Termies: they might be the best in the game because they get a 3++ save. They also get to deepstrike termies which is rare and powerful in 30k.
HQ: Sigismund is one of the best beat stick characters in the game.
4. Night lords:
Infantry: The terror squads are pretty good. So are vets that can infiltrate. They work by outnumbering your opponents and sending in shock troops to run over infantry units. Very much a drop pod legion.
Termies: one of the few legions that get deepstriking termies all the time. So a lot of their tacs revolve around them. Termies can be solid with
Build: I think you would be blending infiltrate and deepstrike. They don't really get to many rules to do other things but they excel at clearing infantry in hth. I think you would be looking to drop pod contemptors, tacs and termies at people. At least, that's how I've seen them played.
5. Iron Hands:
Infantry: -1 to str of any weapon shot at them. so they can't be oblited by anything not a lascannon. Makes 2W models more viable. They also are stubborn in their deployment zone, which is solid if you are playing defensively. To me though the big pick up is that they can take graviton weapons on their infantry. This is a premier weapon in 30k because armored ceremtie on pretty much everything makes meltas much less useful. So you can usually pop spartans on turn, which is an enormous deal.
Termies: Gorgon termies are one of the better deals int he game, plus characters can take cyber eagles to get a 3++ save in termi which is solid. you can also upgrade your praetor to a forgefather which is solid. Termeis can also take graviton guns and they can move an shoot. Totally a great unit.
Contemptors: Blessed Autosimulacra is 10 pts to regain a hp on a 6. This is in addition to the IWND roll from the primarch, so you can run REALLY resilient contemptors.
HQ: Orth is the cheap awesome choice. Autek is a solid beat stick. Ferrus is one of the better multiplier Primarchs.
Build: So this army can do a lot of different builds. Usually tacs hold down the rear. You tend to have a grav unit of some sort as either termies or special weapon support squads. I've seen everything from dread bash to tank hvy with these guys. A VERY strong legion IMO.
6. World Eaters
First off I love the paint scheme. The blue and white with red splatter just gets me.
Infantry: Well this legions is very in your face. You often see just tons of big units running across the board or in spartans.
Termies: Red butcher are brutally cheap and effective.
HQ: Kharn won't disappoint. He's a butcher as is their primarch.
Builds: Pretty straight forward charge at your opponent. Not a very subtle army, but brutally effective in a lot of scenarios.
Infantry: Any unit hit previously by another unit gains re roll to wounds or armor pen. This opens up all sorts of cool tactics. So basically you take small tac squad to paint bulls eyes on units. It even works for units outside of the ultramarine army which makes it crazy powerful. Beyond the standard infantry, there a lot of 2+ save options like breachers with poweraxes. They can build to infantry heavy build but I think you would need some converting to get there.
Termies: Fullmentarus termies are totally cool. the more you take the better their rules get. they are basically termies that can take hvy weaponns. I personally like the autocannon ones that get tank hunter when you take more than 5 of them. It also has good synergy with your Infantry.
Build: It's a newer rule set I haven;t put my mind around yet, but they get a lot of "tactics" with special rules that they can use through out the game especially if you take the primarch. So they are one of the more reactive armies that really change from turn to turn. Feels very space marine.
8. Death Guard:
Infantry: So tacs and hvy weapons squads score. So you can take basically take an entire army of missile launchers :) Plus you can give them flak missiles to take down fliers and augury scanners to shoot deepstrikers when they come in. Flamers can also pick up shred, which is an awesome rules. And they ignore cover when walking through it. Plus they can take rad grenades which lowers other unit toughness in hth. This works really well with something like bikers that can take power axes and oblit T4. So as infantry armies go, this is a really solid one.
Termies: So they have 2 specialized types of termies. They are both good and both cool. They can litterally spam, termies which is cool. You might need to do a little kit bashing, but things like scythes are totally easy to pick up after market these days.
HQ: Morty is about as reslient as you can get.
Builds: Most builds are foot heavy. Usually slanted toward termies or hvy weapons squads with a couple specialty squads through in. They also work really well with contemptors since everyone is walking the same speed.
9. Luna Wolves:
Infantry: So they are focused on being tactical. I don't love the rules. And never really loved the legion.
Termies: Great stats, but way too expensive. They can take pods that can carry them, but really over priced yet again.
I would pass on these guys unless you really liked the legion. they would infuriate me :p
10. Word Bearers:
Infantry: 3d6 ld is a really strong rule as is the overrun rule. In general, i think that tacs squads with chaplains is strong as a choice in general, but you have to figure out how to get them across the board. So usually the tacs end up being small 10 man squad holding down objectives instead, because they don't really have an interesting deployment or movement rules. that said they are good at not running away from objectives and staying put.
Termies: Termies can be VERY powerful in WB armies. Normally you would through them in a spartan or a storm eagle to get across the board. With chaplains with psychic powers they can become pretty crazy deathstars.
HQs: So the HQs are very restrictive. You basically have to take 2 HQ units and one of the will probably be erebus or Zardu. Zardu opens up daemon summoning without allies. Fianlly Lorgar gets to PICK his psychic powers. You can throow invisibility on your termies which is crazy powerful.
Builds: So most builds are a mix of termeis in a spartan and deepstrike vor galbak. But if you can take allies, cultists can be amazing fearless rending mobs or tons of daemons. You could also go heavy jump pack with the ashen circle and Zardu, but that's not really aligned with the minis you just bought. Finially, you need some tank hunting, so you would need to pick somethings up like rapiers platforms, since there are not really anti infantry tank hunting options beyond combi weapons on termies.
Infantry: Are very solid from on moral, but are slow in movement. Flamers get +1 to str which is solid, and meltas become master crafted. Everyone gets move through cover. So you are basically trying to fit in as many flamers and melta as possible.
Termies: first off you get Storm shields for a 3++ and can pick eternal warrior. So you HQ can be crazy resilient. You also get fire drake termies, which are SOOOOO good. The are 2W with 3++ and they score.
Builds: So fire drakes are VERY good, so many of the build have a spartan with firedrakes and shields and medicae to give them FNP. It's an army that lends itself to deathstar builds and it is pretty strong.
Infantry: Everyone gets infiltrate and they have a good chance to go first. So they basically try to go first and alpha strike. Infiltrate makes a couple types of units useful that normally wouldn't be. They can also go pretty heavy flier or drop pods. All this said, the infantry isn't special in anyway, so you have to take advantage of special deployement. They also have some good special units.
Termies: They get furious charge. Which is OK. I like termies in a storm eagle with power axes and combi meltas. Cheap and useful. They can also take rending claws.
Infantry: So you can choose Scout, Infiltrate, Tank Hunters, Counter Attack, Move Through Cover, or Adamantium Will, army wide at the beginning of the game. Plus you can take units from other armies as elites. So you can mix and match all sort of units. They also get a special seeker squad that is quite good. But in general, you don;t get anything else special for infantry, so they are pretty vanilla. they also have a special dagger which is a specialist weapon. So you can combine it with powerfists for another attack or with units from other armies to do weird combos.
Termies: So you can do things like scouting landraiders or infiltrating termies. They don't get anything special beyond that but their are cool combos like counter strike grave warden termies and such.
Builds: The are all over the place. But they have a very good chance of seizing the initiative, so they can be kind alpha strikey. Most builds are infiltrate based though.
The rest of the legions don't have rules yet :) This doesn't mean they are bad!! You can totally build a competitive army with out special rules, and you have a lot of army choices with a praetor and the ROW options.
They get to choose between stubborn or furious charge as a army wide rule. Plus you get to get shiny new minis and rules that everyone will be covetous of when they are released :)